Electric submersible pump configuration

ABSTRACT

A housing-less wellbore pump assembly is described. The pump assembly includes multiple pump stages connected end-to-end axially to pump well fluid in an uphole direction. Each pump stage includes a rotating impeller to rotate to provide kinetic energy to flow fluid through the wellbore pump assembly and a stationary diffuser within which the rotating impeller is positioned. The stationary diffuser converts the kinetic energy received from the rotating impeller to head to flow the fluid through the wellbore pump assembly. The stationary diffuser includes an uphole threaded end and a downhole threaded end to threadedly couple with another, uphole-positioned pump stage and with another, downhole-positioned pump stage, respectively. Threads of the uphole threaded end and the downhole threaded end are formed in directions opposite to a rotational direction of the impeller.

TECHNICAL FIELD

This specification relates to electric submersible pumps (ESPs) for oilfield applications.

BACKGROUND

Some oil and gas wells contain enough pressure for hydrocarbons to rise to the surface without stimulation. In other wells, however, the natural drive energy of the reservoir is not strong enough to push the hydrocarbons to the surface. Consequently, such wells require artificial lift to increase the flow of hydrocarbons from the wells. Even in the wells that initially possessed enough pressure for the hydrocarbons to flow to the surface, the pressure depletes over time and may require artificial lift. Therefore, artificial lift is typically used on most wells at some point during their production life. Artificial lift can be performed by the use of a mechanical device positioned inside the well. The electric submersible pump (ESP) is an example of an artificial lift method for lifting volumes of fluids from wellbores.

SUMMARY

This specification describes technologies relating to pumping well fluids using an electric submersible pump (ESP) without a housing. Certain aspects of the subject matter described here can be implemented as a housing-less wellbore pump assembly. The pump assembly includes multiple pump stages connected end-to-end axially to pump well fluid in an uphole direction. Each pump stage includes a rotating impeller to rotate to provide kinetic energy to flow fluid through the wellbore pump assembly and a stationary diffuser within which the rotating impeller is positioned. The stationary diffuser converts the kinetic energy received from the rotating impeller to head to flow the fluid through the wellbore pump assembly. The stationary diffuser includes an uphole threaded end and a downhole threaded end to threadedly couple with another, uphole-positioned pump stage and with another, downhole-positioned pump stage, respectively. Threads of the uphole threaded end and the downhole threaded end are formed in directions opposite to a rotational direction of the impeller.

This, and other aspects, can include one or more of the following features.

The threads of the uphole threaded end can be formed on an inner surface of the diffuser, and the threads of the downhole threaded end can be formed on an outer surface of the diffuser.

The threads of the uphole threaded end can be formed on an outer surface of the diffuser, and the threads of the downhole threaded end can be formed on an inner surface of the diffuser.

The pump assembly can include a first seal positioned between the uphole threaded end and the downhole threaded end of the other, uphole-positioned pump stage and a second seal positioned between the downhole threaded end and the uphole threaded end of the other, downhole-positioned pump stage.

The pump assembly can include a pump base including a threaded end threadedly coupled to a downhole threaded end of a downhole-most pump stage. Threads of the threaded end of the pump base and the downhole threaded end of the downhole-most pump stage can be formed in directions opposite to the rotational direction of the impeller. The pump assembly can include a seal positioned between the threaded end of the pump base and the downhole threaded end of the downhole-most pump stage.

The downhole-most pump stage can include a diffuser spacer including threaded ends to threadedly couple to the threaded end of the pump base and the downhole threaded end of the downhole-most pump stage. The threads of the diffuser spacer can be formed in directions opposite to the rotational direction of the impeller.

The pump assembly can include a pump head including a threaded end threadedly coupled to an uphole threaded end of an uphole-most pump stage. Threads of the threaded end of the pump head and the uphole threaded end of the uphole-most pump stage can be formed in directions opposite to the rotational direction of the impeller. The pump assembly can include a seal positioned between the threaded end of the pump head and the uphole threaded end of the uphole-most pump stage.

The multiple pump stages can be threadedly connected end-to-end axially without an outer housing.

Certain aspects of the subject matter described here can be implemented as a housing-less wellbore pump assembly. The pump assembly includes a pump base including a threaded end and a pump stage. The pump stage includes a rotating impeller to rotate to provide kinetic energy to flow fluid through the pump stage and the pump base in an uphole direction through a wellbore and a stationary diffuser within which the rotating impeller is positioned. The diffuser converts the kinetic energy received from the rotating impeller to head to flow the fluid through the pump stage. The diffuser includes a downhole threaded end connected with the threaded end of the pump base by a threaded coupling. The threads of the threaded ends of the diffuser and the pump base are formed in directions opposite to a rotational direction of the impeller.

This, and other aspects, can include one or more of the following features.

The pump assembly can include a diffuser spacer positioned between the diffuser and the pump base. The diffuser spacer can include threaded ends connected with the threaded end of the pump base and the downhole threaded end of the diffuser by respective threaded couplings. The threads of the threaded ends of the diffuser spacer, pump base, and diffuser are formed in directions opposite to the rotational direction of the impeller.

The threaded coupling between one of the threaded ends of the diffuser spacer and the threaded end of the pump base can include a seal to block fluid flowing through the pump base and the pump stage from leaking outside the pump stage.

The threaded coupling between the other of the threaded ends of the diffuser spacer and the downhole threaded end of the diffuser can include a seal to block fluid flowing through the pump base and the pump stage from leaking outside the pump stage.

The pump stage can be a first pump. The impeller can be a first impeller. The diffuser can be a first diffuser. The threaded coupling can be a first threaded coupling. The pump assembly can include a second pump stage uphole of the first pump stage. The second pump stage can include a second rotating impeller to rotate to provide kinetic energy to flow the fluid received from the first pump stage through the second pump stage in the uphole direction and a second stationary diffuser within which the second rotating impeller is positioned. The second diffuser can convert the kinetic energy received from the second impeller to head to flow the fluid through the second pump stage. The second diffuser can include a downhole threaded end connected with an uphole threaded end of the first diffuser by a second threaded coupling. The threads of the threaded ends of the first and second diffusers are formed in directions opposite to a rotational direction of the impellers.

The pump assembly can include a pump head including a threaded end. An uphole threaded end of the second diffuser can be connected with the threaded end of the pump head by a third threaded coupling. The threads of the threaded ends of the second diffuser and the pump head are formed in directions opposite to the rotational direction of the impellers.

The threaded coupling between the uphole end of the second diffuser and the threaded end of the pump head can include a seal to block fluid flowing through the pump head and the second pump stage from leaking outside the second pump stage.

The details of one or more implementations of the subject matter described in this specification are set forth in the accompanying drawings and the description. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an electric submersible pump (ESP), according to an implementation.

FIGS. 2A & 2B are views of an example of a threaded coupling between diffusers of the ESP of FIG. 1.

FIGS. 3A & 3B are views of an example of assembling components of the ESP of FIG. 1.

FIGS. 4A & 4B are views of an example of assembling components of the ESP of FIG. 1.

FIG. 5 is a diagram of an example of the ESP of FIG. 1 within a wellbore.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

Some electric submersible pumps (ESPs) can pump approximately 150 to 150,000 barrels per day (bpd) of well fluid from a wellbore. An ESP system can include a centrifugal pump, a protector, a power delivery cable, a motor, and surface controls. The pump can be used to transfer fluid from one location to another. The motor can provide mechanical power to drive the pump, and the power delivery cable can supply the motor with electrical power from the surface. The protector can absorb a thrust load from the pump, transmit power from the motor to the pump, equalize pressure, provide and receive additional motor oil as temperature fluctuates, and prevent well fluid from entering the motor. The pump can include multiple stages of impellers and diffusers. A rotating impeller can add kinetic energy to a fluid, and a stationary diffuser can convert the kinetic energy of the fluid from the impeller into head (or pressure). Pump stages can be stacked in series to form a multi-stage system. In a multi-stage system, the head generated in each stage is summative. For example, the total head developed by a multi-stage system can increase linearly from the first to the last stage. In conventional ESPs, the pump stages can be contained within a housing.

According to the affinity laws for centrifugal pumps, at constant shaft rotation speed, increase in head (that is, pressure) provided by a pump is proportional to the square of the impeller diameter. Therefore, if the impeller diameter is increased, the pump (rotating at the same speed) can provide more head to the fluid it is pressurizing. If the housing of an ESP is removed, the outer diameter of the ESP can be increased while preserving the thickness of the walls of the ESP, and the impellers can increase in size due to the increase in space within the ESP. The technologies described in this specification can preserve structural integrity and sealing capability of an ESP, despite the absence of the outer housing. The diffusers can have threaded ends that tighten in a direction opposite of the rotation of the impellers to mitigate the potential of unthreading and improve assembly integrity of the pump. Based on this assembly, a compression tube may not be necessary because the torque from the threaded connections can provide the resistance to prevent the diffusers from rotating during pump operation. Static seals, such as O-rings can be inserted within grooves between diffusers for additional sealing capability.

Downhole ESPs operate in environments where space is limited radially, and increasing impeller diameters is desirable to increase lift provided to well fluids to be produced to the surface. In another aspect, keeping ESP length short is typically desirable to mitigate bending stress on the pump, especially in the case that a severe dog leg is present. As another example, short pump lengths are also desirable for tandem pumps installed through a lubricator, which can have a fixed length and height. Particular implementations of the subject matter described in this specification can be implemented so as to realize one or more of the following advantages. The pump is housing-less, and larger impellers can be used to generate more lift. The pump can have a shorter length in comparison to a conventional pump that provides the same amount of lift. The mitigation of bending stress can reduce potential rubbing between pump stages and can prevent heat generation and undesirable increases in power usage. Therefore, ESP operational life can be extended, and reliability can be improved, thereby reducing field operating costs and likelihood of deferred production.

FIG. 1 illustrates an example of a housing-less wellbore pump assembly 100. The pump assembly 100 can include a pump head 101 (not to be confused with hydraulic head, which is related to differential pressure), a pump base 103, a pump shaft 151, a head bearing 153, an upper ring 155, a compression nut 157, a lower ring 165, a base bearing 167, and multiple pump stages, for example, three pump stages (180A, 180B, 180C) connected end-to-end axially, which can pump well fluid in an uphole direction. The pump stage (180A, 180B, 180C) can include a rotating impeller (161A, 161B, 161C) to rotate to provide kinetic energy to flow fluid through the assembly 100 and a stationary diffuser (105A, 105B, 105C), within which the impeller (161A, 161B, 161C) is positioned. In certain implementations, the pump assembly 100 can include a diffuser spacer 107 that can be positioned between the diffuser (for example, 105A of the pump stage 180A) and the pump base 103. The diffuser spacer 107 can include threaded ends (131A, 141B) to threadedly couple to the threaded end 141A of the pump base 103 and the downhole threaded end of a diffuser, such as the threaded end 131B of the downhole-most diffuser 105A of pump stage 180A. As another example different from the pump assembly 100 shown in FIG. 1, the pump base 103 can include a threaded end 141A threadedly coupled to a downhole threaded end of a downhole-most pump stage (for example, the threaded end 131B of pump stage 180A). The pump assembly 100 can include additional diffusers apart from those that are a component of a pump stage (180A, 180B, 180C), such as the diffuser 105D. In certain implementations, the pump assembly 100 can include an adapter 109 that can be positioned between the pump head 101 and a diffuser, such as the diffuser 105D. As another example different from the pump assembly 100 shown in FIG. 1, the pump head 101 can include a threaded end 131G threadedly coupled to an uphole threaded end of an uphole-most pump stage. The pump assembly 100 can include additional components, such as diffuser internals or bearings, but have not been shown in FIG. 1.

The components of the pump assembly 100 can be categorized as inner components or outer components. The outer components of the pump assembly 100 include components that share a surface with the outer surface 111 of the assembly 100 and can be in contact with fluids outside of the pump assembly 100, that is, the fluids that do not enter and get pressurized by the pump assembly 100. The outer components of the pump assembly 100 can include, for example, the pump head 101, the pump base 103, the diffuser spacer 107, the adapter 109, and the diffusers 105A, 105B, 105C, 105D. The outer components of the pump assembly 100 can have uphole threaded ends—for example, 131A of diffuser spacer 107; 131B of diffuser 105A; 131C of diffuser 105B; 131D of diffuser 105C; 131E of diffuser 105D; 131F of adapter 109; and 131G of pump head 101. The outer components of the pump assembly 100 can have downhole threaded ends—for example, 141A of pump base 103; 141B of diffuser spacer 107; 141C of diffuser 105A; 141D of diffuser 105B; 141E of diffuser 105C; 141F of diffuser 105D; and 141G of adapter 109. The pump assembly 100 can include seals between the outer components—for example, 121A between pump base 103 and diffuser spacer 107; 121B between diffuser spacer 107 and diffuser 105A; 121C between diffusers 105A and 105B; 121D between diffusers 105B and 105C; 121E between diffusers 105C and 105D; 121F between diffuser 105D and adapter 109; and 121G between adapter 109 and pump head 101.

The inner components of the pump assembly 100 include components that are positioned within the outer components and can interact with the fluid that enters and gets pressurized by the pump assembly 100. The inner components of the pump assembly 100 can include, for example, the pump shaft 151, the head bearing 153, the upper ring 155, the compression nut 157, the lower ring 165, the base bearing 167, and the impellers 161A, 161B, 161C. In certain implementations, portions of the outer components can also interact with the fluid that enters and gets pressurized by the pump assembly 100—for example, the inner surfaces of the diffusers (105A, 105B, 105C, 105D).

The impellers (161A, 161B, 161C) of the pump assembly 100 can be mechanically coupled to the pump shaft 151. The pump shaft 151 can be connected to and rotated by a motor (refer to FIG. 5), thereby causing the impellers (161A, 161B, 161C) to rotate. The diffuser (105A, 105B, 105C) can convert the kinetic energy received from the corresponding rotating impeller (161A, 161B, 161C) to head (that is, hydraulic head) to flow the fluid through the pump stage (180A, 180B, 180C) and the assembly 100. In other words, the rotating impeller (161A, 161B, 161C) of the pump stage (180A, 180B, 180C) can rotate to provide kinetic energy to flow fluid through the pump stage (180A, 180B, 180C) and the pump base 103 in an uphole direction through a wellbore. The pump assembly 100 can include an impeller spacer (163A, 163B, 163C) uphole of each impeller (161A, 161B, 161C). The compression nut 157 can connect the last, uphole impeller spacer (for example, 163C) to the pump shaft 151. The upper ring 155 and lower ring 165 (also referred to as two-piece rings or split rings) can join the rotating components of the assembly 100 to the pump shaft 151 to transmit power and axial thrust. The head bearing 153 and the base bearing 167 constrains the relative motion of the shaft 151 to only the desired motion, for example, free rotation around the axis of the shaft 151.

After initial assembly, gaps may exist between the impellers (such as 161A, 161B, 161C) and the impeller spacers (such as 163A, 163B, 163C). The impellers (161A, 161B, 161C) and impeller spacers (163A, 163B, 163C) can be supported by the bottom by the lower ring 165, which can be locked onto the pump shaft 151. The compression nut 157 can rigidly hold the assembled impellers and impeller spacers axially against the lower ring 165, so that the initial gaps are reduced or removed. The two-piece rings (155, 165) can ensure that the assembled impellers (161A, 161B, 161C), impeller spacers (163A, 163B, 163C), and compression nut (157) are locked onto the pump shaft 151 and can prevent axial movement along the shaft 151. In other words, the two-piece rings (155, 165) can prevent the assembled components from sliding along the pump shaft 151. Preventing axial movement of the components along the pump shaft 151 can ensure that any axial thrust is transmitted through the pump shaft 151. Additional friction can cause heat generation and can even result in potential pump failure. Preventing axial movement of the components along the pump shaft 151 can also prevent potential rubbing contact (axially) between the rotating components.

The outer surface 111 of the pump assembly 100 can have varying diameters across the axial length of the pump assembly 100, but the outer surface 111 across the diffusers (105A, 105B, 105C) of the pump stages (180A, 180B, 180C) can be uniform. In this specification, “substantially” means a deviation, allowance, or variation from a mentioned value is within the tolerance limits of any machinery used to manufacture the part. Because the pump assembly 100 is housing-less, the outer surface 111 can have a larger diameter in comparison to conventional pump assemblies that have housings. Without the housing and preserving thickness of the diffusers (105A, 105B, 105C) of the pump stages (180A, 180B, 180C), the incremental space available for the tip of the impeller (161A, 161B, 161C) of each pump stage (180A, 180B, 180C) to occupy can increase, in some examples, by substantially 0.5″. An increase in impeller diameter can result in a higher developed head (that is, increased pressure differential between the inlet and outlet of the ESP). In one example, a difference between the outer diameter of each pump stage (180A, 180B, 180C), and the diameter of the tip of the impeller 161A, 161B, 161C) of each pump stage (180A, 180B, 180C) can be between 0.2″ and 0.4″. In comparison to a pump assembly that has a housing with a similar outer diameter, the generated head for a housing-less pump described here can increase by approximately 23% attributable to the possible increase in impeller diameter by approximately 0.5″. In another example, a difference between the outer diameter of each pump stage (180A, 180B, 180C) and the diameter of the tip of the impeller (161A, 161B, 161C) of each pump stage (180A, 180B, 180C) can be between 0.1″ and 0.2″. In comparison to a pump assembly that has a housing with a similar outer diameter, the generated head can increase by approximately 32% attributable to the possible increase in impeller diameter by approximately 0.5″.

The threaded ends can be connected with each other by threaded couplings. The threads of the threaded ends can be formed in directions opposite to a rotational direction of the impellers. For example, in response to the impeller 161A rotating to provide the kinetic energy to flow the fluid through the pump stage 180A, the threaded coupling between the diffuser spacer 107 and the pump base 103 can prevent the components (107 and 103) from disassembling because the threads of the threaded couplings are formed in an opposite direction from the rotation of the impeller 161A. Similarly, in response to the impeller 161A rotating to provide the kinetic energy to flow the fluid through the pump stage 180A, the threaded coupling between the diffuser spacer 107 and the diffuser 105A can prevent the components (107 and 105A) from disassembling.

In some implementations (contrasting from the example shown in FIG. 1), the threads of the uphole threaded end of a diffuser are formed on the outer surface of the diffuser, and the threads of the downhole threaded end of a diffuser are formed on the inner surface of the diffuser. The various components of the pump assembly 100 that are threaded can have threads formed on an inner surface or an outer surface of the component. For example, as shown in FIG. 1, the pump base 103 can have threads that are formed on its outer surface, which means the spacer 107 has threads that are formed on its inner surface. Optionally (not shown), the pump base 103 can have threads that are formed on its inner surface, which means the spacer 107 has threads that are formed on its outer surface. The thread form (that is, the cross-sectional shape of the threads) can be any shape, such as square, triangular, or trapezoidal. For example, the threads can be vee-threads or buttress threads, which have a triangular form. Other properties of the threads (such as thread angle, major and minor diameters, and thread pitch) can depend on the type of thread used and the overall size of the pump assembly 100.

The threads of the uphole threaded end and the downhole threaded end can be formed in directions opposite to a rotational direction of the impeller. For example, the threads of the uphole threaded end 131D and the threads of the downhole threaded end 131C of diffuser 105B are formed in a direction opposite to the rotational direction of the impeller 161B. Conventional ESPs typically include a housing to which the pump head and the pump base are threadedly coupled, and conventional EPSs do not typically have threaded ends to couple diffusers together. The pump assembly 100 does not include a housing. In certain implementations, the diffuser spacer 107 includes threaded ends (131A, 141B) connected with the threaded end 141A of the pump base 103 and the downhole threaded end 131B of the diffuser 105A by respective threaded couplings. The threads of the threaded ends (131A, 131B, 141B) can be formed in an opposite direction from the direction of the impeller 161A rotating to provide the kinetic energy to flow the fluid through the pump stage 180A. In certain implementations (not shown in the figures, but using like elements), there are only two pump stages, for example, pump stages 180A and 180B. In the case with two pump stages, the pump head 101 can include a threaded end 131G, and the uphole threaded end 141D of the diffuser 105B can be connected with the threaded end 131G of the pump head 101 by a threaded coupling. The threads of the threaded ends (131G and 141D) can be formed in an opposite direction from the direction of the impeller 161B rotating to provide the kinetic energy to flow the fluid through the pump stage 180B. In certain implementations, there are three pump stages (180A, 180B, 180C), and there is an adapter 109 positioned between the pump head 101 and the uphole-most pump stage (180C). The pump assembly 100 can optionally include an additional diffuser 105D (which is not directly associated with a pump stage) between the adapter 109 and the uphole-most pump stage (180C). The pump head 101, the adapter 109, and the diffuser 105D can include threaded ends and can be threadedly coupled to each other. The threads of the threaded ends of the components of the pump assembly 100 can be formed in an opposite direction of the impellers (such as 161A, 161B, 161C) rotating to provide the kinetic energy to flow the fluid through the pump assembly 100.

This configuration (that is, the tightening of threads between components being opposite the direction of the rotation of the impellers) can improve the assembly integrity and sealing capability of the pump assembly 100. In other words, the pump stages of the pump assembly 100 can be threadedly connected end-to-end axially without an outer housing. In the case that the downhole-most pump stage 180A is threadedly coupled to the pump base 103, the threads of the threaded end 141A of the pump base 103 and the downhole threaded end 131B of the downhole-most pump stage 180A can be formed in directions opposite to the rotational direction of the impellers, such as 161A. In the case that the uphole-most pump stage 180C is threadedly coupled to the pump head 101, the threads of the threaded end 131G of the pump head 101 and the uphole threaded end 141E of the uphole-most pump stage 180C can be formed in directions opposite to the rotational direction of the impeller, such as 161C. Still referring to FIG. 1, in the case that the pump assembly 100 includes an adapter 109 in between and threadedly coupled to the pump head 101 and the uphole-most diffuser 105D, the threads of the threaded ends (131F, 141G) of the adapter 109 can be formed in directions opposite to the rotational direction of the impellers, such as 161C. In the case that the downhole-most pump stage 180A is threadedly coupled to the diffuser spacer 107, the threads of the threaded ends (131A, 141B) of the diffuser spacer 107 can be formed in directions opposite to the rotational direction of the impellers, such as 161A.

FIGS. 2A and 2B show different views of the pump assembly 100 shown in FIG. 1 and illustrate the threaded coupling between diffusers 105B and 105C. During assembly, the components are mechanically threaded together in a direction opposite of the impeller rotation. By assembling the pump assembly 100 in this manner, contact force can be provided between the components. Having the thread-tightening direction opposite of the normal impeller rotation can also provide additional operational integrity by mitigating the risk of unthreading or spinning diffuser effect during pump operation. In other words, the opposite direction of threading and impeller rotation can ensure that the components remain locked together during pump operation.

Each stationary diffuser can include an uphole threaded end to threadedly couple with another pump stage or spacer that is positioned uphole. For example the uphole threaded end 141D of diffuser 105B can couple the diffuser 105C (of pump stage 180C) to the diffuser 105B (of pump stage 180B). Each stationary diffuser can also include a downhole threaded end to threadedly couple with another pump stage or spacer that is positioned downhole. For example, the downhole threaded end 131D of diffuser 105C can couple the diffuser 105B of (pump stage 180B) to the diffuser 105C (of pump stage 180C). In certain implementations, the threads of the uphole threaded end of a diffuser are formed on the inner surface of the diffuser (for example, threaded end 141D of diffuser 105B), and the threads of the downhole threaded end of a diffuser are formed on the outer surface of the diffuser (for example, threaded end 131D of diffuser 105C).

Referring to FIGS. 3A, 3B, 4A, and 4B, the various components of the pump assembly 100 can include grooves upon which the seals can be installed. The pump assembly 100 can include seals, such as O-rings or other seals, between pump stages to improve sealing capability. A first seal can be positioned between the uphole threaded end of a pump stage and the downhole threaded end of another pump stage positioned uphole. For example, referring back to FIGS. 2A and 2B, the seal 121D is positioned between the uphole threaded end 141D of pump stage 180B and the downhole threaded end 131D of pump stage 180C. A second seal can be positioned between the downhole threaded end of a pump stage and the uphole threaded end of another pump stage positioned downhole. For example, the seal 121C is positioned between the downhole threaded end 131C of pump stage 180B and the uphole threaded end 141C of pump stage 180A. The pump assembly 100 can include seals, such as O-rings, between other components, as well. The threaded end of a component (such as 141A of 103) can include one or more grooves, and the grooves can be located uphole relative to the threading (FIGS. 3A and 3B), downhole relative to the threading (FIGS. 4A and 4B), or both (if there are multiple grooves on one threaded end). On one hand, having multiple seals for a single threaded coupling can improve sealing reliability. On the other hand, the presence of multiple seals can increase the chances of friction (that is, rubbing) during the assembly process. As one example, one of the components that are threadedly coupled together can include a groove on an inner surface for a seal to be installed between them (for example, 121B between 131B and 141B as shown in FIGS. 3A and 3B). Optionally, one of the components that are threadedly coupled together can include a groove on an outer surface for a seal to be installed between them (not shown). In certain implementations, the seal (for example, O-ring) is installed into the groove first, and then the two components (such as two diffusers) can be threadedly coupled.

Referring back to FIG. 1 for example, the threaded coupling between one of the threaded ends (131A) of the diffuser spacer 107 and the threaded end 141A of the pump base 103 can include a seal, such as the seal 121A, that can block fluid flowing through the pump base 103 and the pump stage 180A from leaking outside the pump stage 180A. Similarly, the threaded coupling between the other of the threaded ends (141B) of the diffuser spacer 107 and the downhole threaded end 131B of the diffuser 105A can include a seal, such as the seal 121B, that can block fluid flowing through the pump base 103 and the pump stage 180A from leaking outside the pump stage 180A. As another example (not shown in the figures, but using like elements), in the case of the pump assembly 100 having only two pump stages, such as pump stages 180A and 180B, the threaded coupling between the uphole end 141D of the diffuser 105B and the threaded end 131G of the pump head 101 can include a seal, such as the seal 121G, that can block fluid flowing through the pump head and the pump stage 180B from leaking outside the second pump stage 180B. Referring back to FIG. 1, in the case that the pump assembly 100 includes an adapter 109 in between and threadedly coupled to the pump head 101 and the uphole-most diffuser 105D, the threaded coupling between the uphole threaded end 141G of the adapter 109 and the downhole threaded end 131G of the pump head 101 can include a seal, such as seal 121G, that can block fluid flowing through the pump assembly 100 from leaking outside the pump assembly 100.

FIG. 5 illustrates an example of a production system 500 within a wellbore. The production system 500 can include a casing 502, a motor 503, a protector 504, a pump intake 505, a packer 506, production tubing 507, and a pump assembly, such as the housing-less pump assembly 100 shown in FIG. 1. The various components of the production system 500 can have substantially the same outer diameter with a variation as described earlier. In certain implementations, the components of the production system 500 can have different diameters, but all components can be designed to handle a desired flow of well fluid 501. In the particular examples described in this specification, the pump assembly 100 lifts well fluid 501 in an uphole direction, that is, toward a surface of the wellbore. As shown in FIG. 5, the motor 503 can be positioned downhole relative to the pump assembly 100. The order of components of a wellbore production system can vary, but the intake 505 is located downhole of the pump assembly 100, and the protector 504 is typically located adjacent to the motor 503. For example, the protector 504 can be positioned between the pump assembly 100 and the motor 503 and can absorb a portion of axial loads from the pump assembly 100 pressurizing the well fluid 501.

The packer 506 can be positioned uphole relative to the pump assembly 100 and can fluidically isolate a portion of the wellbore downhole relative to the pump assembly 100 from a remainder of the wellbore uphole relative to the pump assembly 100. For example, the packer 506 can be positioned to isolate the reservoir, such that any fluid from the reservoir first flows through the pump assembly 100 before entering the production tubing 507 and traveling further uphole. The pump intake 505 can include a screen to filter debris before fluid enters the pump assembly 100. The motor 503 can be a center-tandem (CT) motor or other suitable motor. The production system 500 can include additional components, such as downhole sensors, for example, for pressure, temperature, flow rate, or vibration; additional packers; wellheads; centralizers or protectorlizers; check valves; motor shroud or recirculation systems; additional screens or filters; or a bypass, for example, a Y-tool.

Well fluid 501 can flow from the reservoir and enter the casing 502 through perforations or other openings and travel in an uphole direction. Well fluid 501 can flow past the motor 503 and protector 504, and the flow of well fluid 501 can provide cooling to the motor 503. The well fluid 501 can flow into the pump intake 505 and through the vanes (or impellers) of the pump assembly 100. The pump assembly 100 can pressurize the well fluid 501, for example, in order to lift the well fluid 501 to the surface through the production tubing 507. Although the production system 500 is shown in FIG. 5 as being located within a wellbore, the production system 500 can optionally be located and operated at the surface. The production system 500 can have a horizontal orientation, and in this case downhole is analogous to upstream, and uphole is analogous to downstream.

Thus, particular implementations of the subject matter have been described. Other implementations are within the scope of the following claims. In some cases, the actions recited in the claims can be performed in a different order and still achieve desirable results. In addition, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. 

What is claimed is:
 1. A housing-less wellbore pump assembly comprising: a plurality of pump stages connected end-to-end axially and configured to pump well fluid in an uphole direction, each pump stage comprising: a rotating impeller configured to rotate to provide kinetic energy to flow fluid through the wellbore pump assembly; and a stationary diffuser within which the rotating impeller is positioned, the stationary diffuser configured to convert the kinetic energy received from the rotating impeller to head to flow the fluid through the wellbore pump assembly, the stationary diffuser comprising an uphole threaded end and a downhole threaded end configured to threadedly couple with another, uphole-positioned pump stage and with another, downhole-positioned pump stage, respectively, threads of the uphole threaded end and the downhole threaded end formed in directions opposite to a rotational direction of the impeller.
 2. The pump assembly of claim 1, wherein the threads of the uphole threaded end are formed on an outer surface of the diffuser, and wherein the threads of the downhole threaded end are formed on an inner surface of the diffuser.
 3. The pump assembly of claim 1, wherein the threads of the uphole threaded end are formed on an inner surface of the diffuser, and wherein the threads of the downhole threaded end are formed on an outer surface of the diffuser.
 4. The pump assembly of claim 3, further comprising: a first seal positioned between the uphole threaded end and the downhole threaded end of the other, uphole-positioned pump stage; and a second seal positioned between the downhole threaded end and the uphole threaded end of the other, downhole-positioned pump stage.
 5. The pump assembly of claim 1, further comprising: a pump base comprising a threaded end threadedly coupled to a downhole threaded end of a downhole-most pump stage, threads of the threaded end of the pump base and the downhole threaded end of the downhole-most pump stage formed in directions opposite to the rotational direction of the impeller; and a seal positioned between the threaded end of the pump base and the downhole threaded end of the downhole-most pump stage.
 6. The pump assembly of claim 5, wherein the downhole-most pump stage comprises a diffuser spacer comprising threaded ends to threadedly couple to the threaded end of the pump base and the downhole threaded end of the downhole-most pump stage, wherein threads of the diffuser spacer formed in directions opposite to the rotational direction of the impeller.
 7. The pump assembly of claim 1, further comprising: a pump head comprising a threaded end threadedly coupled to an uphole threaded end of an uphole-most pump stage, threads of the threaded end of the pump head and the uphole threaded end of the uphole-most pump stage formed in directions opposite to the rotational direction of the impeller; and a seal positioned between the threaded end of the pump head and the uphole threaded end of the uphole-most pump stage.
 8. The pump assembly of claim 1, wherein the plurality of pump stages are threadedly connected end-to-end axially without an outer housing.
 9. A housing-less wellbore pump assembly comprising: a pump base comprising a threaded end; and a pump stage comprising: a rotating impeller configured to rotate to provide kinetic energy to flow fluid through the pump stage and the pump base in an uphole direction through a wellbore; and a stationary diffuser within which the rotating impeller is positioned, the diffuser configured to convert the kinetic energy received from the rotating impeller to head to flow the fluid through the pump stage, the diffuser comprising a downhole threaded end connected with the threaded end of the pump base by a threaded coupling, threads of the threaded ends of the diffuser and the pump base formed in directions opposite to a rotational direction of the impeller.
 10. The pump assembly of claim 9, further comprising a diffuser spacer positioned between the diffuser and the pump base, the diffuser spacer comprising threaded ends connected with the threaded end of the pump base and the downhole threaded end of the diffuser by respective threaded couplings, threads of the threaded ends of the diffuser spacer, pump base, and diffuser formed in directions opposite to the rotational direction of the impeller.
 11. The pump assembly of claim 10, wherein the threaded coupling between one of the threaded ends of the diffuser spacer and the threaded end of the pump base comprises a seal configured to block fluid flowing through the pump base and the pump stage from leaking outside the pump stage.
 12. The pump assembly of claim 10, wherein the threaded coupling between the other of the threaded ends of the diffuser spacer and the downhole threaded end of the diffuser comprises a seal configured to block fluid flowing through the pump base and the pump stage from leaking outside the pump stage.
 13. The pump assembly of claim 9, wherein the pump stage is a first pump stage, the impeller is a first impeller, the diffuser is a first diffuser, the threaded coupling is a first threaded coupling wherein the pump assembly further comprises a second pump stage uphole of the first pump stage, the second pump stage comprising: a second rotating impeller configured to rotate to provide kinetic energy to flow the fluid received from the first pump stage through the second pump stage in the uphole direction; and a second stationary diffuser within which the second rotating impeller is positioned, the second diffuser configured to convert the kinetic energy received from the second impeller to head to flow the fluid through the second pump stage, the second diffuser comprising a downhole threaded end connected with an uphole threaded end of the first diffuser by a second threaded coupling, threads of the threaded ends of the first and second diffusers formed in directions opposite to a rotational direction of the impellers.
 14. The pump assembly of claim 13, further comprising a pump head comprising a threaded end, wherein an uphole threaded end of the second diffuser connected with the threaded end of the pump head by a third threaded coupling, threads of the threaded ends of the second diffuser and the pump head formed in directions opposite to the rotational direction of the impellers.
 15. The pump assembly of claim 14, wherein the threaded coupling between the uphole end of the second diffuser and the threaded end of the pump head comprises a seal configured to block fluid flowing through the pump head and the second pump stage from leaking outside the second pump stage. 